The clock_getcpuclockid() function returns the clock ID of
the CPU-time clock of the process specified by pid. If the process
described by pid exists and the calling process has permission,
the clock ID of this clock is stored in clock_id.

If pid is zero, the clock ID of the CPU-time clock of the process
marking the call is returned in clock_id.

A process always has permission to obtain the CPU-time clock ID of another process.